This invention relates to nickel-based and cobalt-based superalloys, and, more particularly, to coatings that are applied to such superalloys to protect them from oxidation and corrosion damage.
Aircraft gas turbine (jet) engines operate by drawing air into the front end of the engine, compressing it, mixing the compressed air with fuel, and igniting the mixture in a combustor to form a hot exhaust gas. The exhaust gas passes through a turbine which drives the air compressor, and exits the back end of the engine to generate thrust that drives the engine and aircraft forward. Those portions of the engine contacted by the hot exhaust gas are repeatedly cycled to both high temperatures and high stresses during service of the engine.
There is a continuing effort to achieve ever-higher operating temperatures in the combustor, turbine, and exhaust sections of the engine, because higher operating temperatures lead to increased engine efficiency. At the present time, operating temperatures can exceed 2000.degree. F. One primary approach to the attaining of increased operating temperatures has been the introduction and refinement of nickel-based and cobalt-based alloys, termed "superalloys" in the art, that are strong at high operating temperatures and resist progressive deformation called creep and cyclic deformation called fatigue during extended service.
Although the superalloys exhibit excellent mechanical properties at elevated temperatures, they are subject to severe degradation by oxidation and hot corrosion during service. The hot exhaust gases, which may include large amounts of salts as well as combustion products, are particularly damaging to the metal alloys, and can quickly corrode and erode the metal away. Many parts of the hot sections of the engine require the maintaining of tight tolerances in order to be effective, and loss of substantial amounts of metal by hot gas corrosion and erosion leads to reduction of engine performance.
Since hot corrosion and erosion occur primarily at surfaces exposed to the hot combustion gas, a number of different types of surface treatments have been developed to resist the surface damage. In one approach, a coating is applied to the surface. The coating material is selected to be resistant to hot oxidation, corrosion, and erosion. It also must be selected and applied in such a way as to resist cracking and flaking away (termed "spalling") during repeated thermal cycles from ambient temperature to the operating temperature.
One class of protective coating, termed a "thermal barrier coating" or "TBC" system in the art, is formed of one, two or more layers, one on top of the other in the multilayer cases. In one TBC system having two layers, the bottom layer or bond coat adjacent the superalloy substrate to be protected is an MCrAlY alloy. The top layer or top coat is a ceramic, typically formed of modified zirconium oxide that resists erosion damage and also insulates the superalloy substrate.
A number of techniques for applying the thermal barrier coating system to an article are known in the art. In one such approach presented as an example, the bond coat is applied by a pack cementation process in which the surface of the article is contacted at elevated temperature to a mixture of inert particles, a small amount of an aluminum-containing alloy, and a halide activator material. The bond coat is formed by interdiffusion of the aluminum from the aluminum-containing alloy and the nickel in the article, after which the upper surface of the bond coat is oxidized. Plasma spray processes are also sometimes used to deposit the bond coat. The ceramic top coat layer is applied by physical vapor deposition or other ceramic deposition technique such as plasma spray deposition.
Thermal barrier coating systems work well, and achieve good performance in a variety of hot section applications.
A problem arises, however, because once the thermal barrier coating system is in place on the surface of an article, it is difficult to coat other portions of the surface with an aluminide. Such a situation arises in various contexts. Thus, for example, if an article is first coated with the thermal barrier coating and there arises a need to drill a hole in the article or machine away a small region of the surface, the result is an exposed portion that has no coating protection. In another example, if an article having a thermal barrier coating system in place is joined to another article, the region near the joint has no protective coating. Finally, in a common situation the thermal barrier coating may be damaged during use, and a repair is necessary to a relatively small area of the article.
If the conventional pack cementation approach to applying the bond coat is used on an article that already has a thermal barrier coating system in place over another portion of the surface, the existing TBC material is observed to crack so that little or no service life remains in the thermal barrier coating. If that portion is repaired, then other regions of the TBC coating similarly fail.
There is a need for an approach for applying an aluminide coating layer to an article which has an existing TBC system in place over some other portion of the surface of the article. The approach must be compatible with existing manufacturing and/or repair techniques and must produce an acceptable nickel or cobalt aluminide coating on the surface of the article. The present invention fulfills this need, and further provides related advantages.